Ultra-long-haul (ULH) optical transmission is of crucial importance to increase the flexibility of future optical networks. The transmission distance of ULH transmission is limited by amplified spontaneous emission (ASE) noise and fiber nonlinearities. The use of dispersion-managed solitons (DMS) and other signal formats in such systems has attracted attention because of the potential for increased transmission performance. DMS systems balance self-phase-modulation (SPM) with fiber dispersion, and avoid intra-channel cross-phase-modulation (XPM) and four-wave-mixing (FWM) by maintaining a moderate degree of pulse breathing. However, there exists a severe nonlinear penalty in DMS-based dense-wavelength-multiplexed (DWDM) transmissions, namely inter-channel XPM which introduces severe timing jitter.
Differential-phase-shift-keying (DPSK) and other phase-shift keying modulation formats such as quadrature phase-shift keying (DQPSK) have also attracted much attention because of their potential to significantly reduce the XPM penalty in DWDM systems. However, the performance of such systems is generally limited by the Gordon-Mollenauer effect, in which ASE power noise is converted into phase noise by self-phase modulation (SPM).
Nonlinearity management based on distributed nonlinearity compensation (nonlinearity compensated at several locations within a soliton period) has been proposed to improve transmission performance. A distributed nonlinear compensator is, however, extremely complicated, and therefore very expensive. An example of distributed nonlinearity compensation includes fibers with alternating positive and negative nonlinear refractive indices (n2). Such fibers can be used to effectively cancel the nonlinear phase shift resulting from SPM. However, such fiber does not exist at the communication wavelength window (˜1.55 um). Accordingly, a need exists for a practical and cost effective method and apparatus for reducing the nonlinear phase noise resulting from SPM and ASE.